The silver halide photographic materials and methods for forming images using these materials which are available at the present time are useful in many and various fields. The halogen composition of the silver halide emulsions used in many of these photosensitive materials often include silver iodobromide, silver chloroiodobromide or silver bromochloride, and other silver halides based principally on silver bromide, in order to achieve the required high speeds.
On the other hand, with the products which are used in markets where there is a great demand for finishing large numbers of prints in a short period of time, such as the color printing paper type photosensitive materials, silver bromide or silver chlorobromide which is substantially silver iodide free is used because of the need to realize high processing speeds.
In recent years, the demand for increased processing speeds in connection with color printing papers has increased, and much research has been done in this connection. Thus it is well known that the development rate can be greatly increased by raising the silver chloride content of the silver halide emulsion which is being used.
However, silver halide emulsions which have a high silver chloride content are liable to fogging and it is difficult to achieve high speeds with normal chemical sensitization with these emulsions. Further, they are known to suffer from problems with reciprocity law failure which causes, for example, changes in speed and gradation depending on the exposure luminance.
Various techniques have been developed with a view to overcoming the disadvantages of the silver halide emulsions which have a high silver chloride content as described above.
Thus it is indicated in JP-A-58-95736, U.S. Pat. No. 4,564,591 (JP-A-58-108533), JP-A-61-222844 and U.S. Pat. No. 4,590,155 (JP-A-60-222845) (the term "JP-A" as used herein means an "unexamined published Japanese patent application") that the provision of silver halide grain structures such that there is a layer or phase which has a high silver bromide content is effective for overcoming the disadvantages of silver halide emulsions which have a high silver chloride content. Thus, the introduction of a layer or phase which has a high silver bromide content has various effects on the photographic performance of a silver halide emulsion which has a high silver chloride content, but it has little improving effect in terms of reciprocity law failure.
It is also known that the doping of silver halide grains with iridium is effective for improving a silver halide emulsion in respect of reciprocity law failure. For example, in JP-B-43-4935 (the term "JP-B" as used herein means "examined Japanese patent publication") it is indicated that images which have almost constant gradation can be obtained over a wide range of exposure times with photographic materials in which a trace amount of an iridium compound has been added during the precipitation or ripening of the silver halide emulsion. However, it is indicated on page 201 of volume 33 of the Journal of Photographic Science by Twikkey that latent image intensification occurs during a comparatively short interval of time from 15 seconds to about 2 hours after exposure in the case of iridium doped silver halide emulsions which have a high silver chloride content. For example, changes inevitably occur in the photographic performance as a result of changing the time interval between exposure and processing as a result of this effect and this is undesirable in practice with photosensitive materials which are to be used as color printing papers.
Examples of the iridium doping of silver chloroiodobromide emulsions which have a comparatively high silver chloride content have been disclosed in U.S. Pat. No. 4,126,472 (JP-A-50-116025), JP-A-56-25727, U.S. Pat. No. 4,469,783 (JP-A-58-211753), JP-A-58-215641, U.S. Pat. No. 4,621,041 (JP-A60-19141) and JP-A-61-47941, but in none of these cases is the aforementioned problem of reciprocity law failure overcome.